If its a quad core then its already 64 bit capable. Have a look a getting a 64 bit OS and 8 gigs in there (DDR2 RAMS cheap now, and getting 16 gig of DDR3 for the i7 will cost a heck of a lot depending on the MB dimm slot availablilty) You could be up and running 64 bit with 8gig for about $100 (less the OS but you can always use that on your new PC too) and let the i7's come cheaper in price and save money in the long run (thats what i'm doing, letting the i7's settle in a bit and letting the price drop before going to em)

When I was running 32 bit with 4 gig (with the 3 gig switch) I think I could get to about 6-8 mil polys in Z per mesh but at that it started to become slow.

I also didnt see any stability difference4 between 08 and 09 on the 32 bit side, or even on the 64 bit for that matter.